In recent years, the application of permanent magnet DC motors as prime movers for machine tool feed drives have almost completely replaced the hydraulic motor. As experience was gained with the DC motor for this application, it was discovered that the method used for overload protection was ultraconservative. For certain duty cycles, the overload protection would shut the drive motor off, even though the motor temperature was hardly above the ambient temperature.
The reason for this is due to the difference in the dynamic thermal characteristics between the motor and the overload relay or heater. The motor has a much high thermal inertia than the heater. Therefore, the temperature rise time (thermal time constant) of the motor is much longer than that of the heater. As a result, when a current is applied to the motor and heater in excess of their continuous ratings, the heater temperature will rise rapidly in comparison to the motor, causing the motor circuit to open prematurely; long before the motor begins to get hot.